The invention relates to an electrophotographic or ionographic printer with variable printing speed, including a chargeable image drum, particularly a laser or LED printer for imprinting a medium. A charging unit is provided for charging the image drum and also a toner unit for applying toner to the image drum. The toner unit is arranged in the direction of rotation of the image drum at a predetermined angular distance behind the charging unit. Further, a writing unit is provided by which a beam is moved across the image drum for changing the electric charge on the image drum and there is also a sensor for detecting the charge of the image drum.
Such printers are well known in the art. They achieve an optimal printed image by appropriately tuning the parameters of the printer to one another. In particular, the ratio of the surface charge of the image drum at the deposit location of the toner from the toner unit onto the image drum to the voltage (BIAS) of the toner unit is adjusted according to the given parameters. Furthermore, the voltage of the writing unit must be so selected that a sharp image without ghost images is obtained. The value adjustments are always also dependent on certain ambient conditions.
When the ambient conditions change, the value adjustments or settings are no longer optimal so that the image quality deteriorates. This is also true for a change in the printing speed. It is therefore difficult to obtain a perfect image with changing ambient conditions and changing printing speeds.
In order to obtain an optimal image all parameters must be re-adjusted whenever the ambient conditions or the printing speed change. It is therefore problematic to change the speed of the image drum and, consequently, the printing speed. Printers with variable printing speed therefore generally do not provide optimal images.
It is known to determine the charge of the image drum to determine, in this way, the influence of the ambient conditions and to compensate for this influence by an adjustment to the voltage of the charging unit. The sensor for determining the image drum charge is arranged between the toner unit and the charging unit.
DE 43 36 690 A1 discloses an arrangement for measuring electrical potential contrasts on surfaces, which have a charge image wherein, by means of a measuring probe, an improved determination of the surface potentials is to be achieved and targeted changes of the surface potential are to be effected. To this end, the surface area is used as measuring area and is provided with selected charge patterns and the measuring probe is cutting-edge shaped and extends parallel to an edge of the charge pattern and normal to an assumed direction of movement.
As typical application example for the edge shaped measuring probe an electrophotographic printer is mentioned which includes a charging unit and a developer drum. Although the publication mentions that the measuring probes may be part of the developer station or, respectively, may be represented by the developer drum the printed publication includes no indication as to the effects of such an arrangement. Rather, the publication suggests as practical application an electrophotographic arrangement without developer station. The publication includes no hints to any compensation for changed ambient conditions or changing printing speeds.
Furthermore, Patent Abstracts of Japan, Vol. 006 No. 168 (P-139), 2. September 1982 (Sep. 9, 1982) and JP 57 086842 A (RICOH Co. LTD), May 31, 1982 (May 31, 1982) disclose an electrophotographic printer arrangement wherein detectors are arranged at the edges of an image drum by which the voltage of a corona generated is sensed. The corona generator is moved by a motor in axial direction until the two electrodes measure the same voltage.
U.S. Pat. No. 5,659,841 discloses an electrophotographic printing device which includes a sensor for determining the surface potential of an image drum between a first developer unit and a second developer unit.
Also, U.S. Pat. No. 3,788,739 discloses an electrophotographic printing device wherein a sensor for determining the surface potential of an image drum is arranged adjacent a writing unit. The writing unit and, consequently, the sensor are arranged, in the direction of rotation of the image drum, ahead of a developing unit.
Although with the detection of the charge of the image drum also the influence of changing ambient conditions can at least partially compensated the achievable compensation is not fully satisfactory.
It is therefore the object of the present invention to provide a printer of the type referred to above, that is a printer with variable printing speed, which delivers a perfect image also with changing ambient conditions and different printing speeds.
In a printer with an electrically chargeable image drum, such as a laser or an LED printer, for imprinting a medium by transferring toner to the medium, including, arranged around the image drum, a charging unit, a writing unit and a toner unit, the toner unit is disposed at a predetermined angular distance from the charging unit and a sensor is disposed at essentially the same angular distance from the charging unit to determine the charge of the image unit at the toner location for adjusting the voltage of at least one of the charging unit and the toner unit depending on the charge voltage of the image drum as determined by the sensor.
However the charge of the image drum at the predetermined angular distance from the charge unit can be determined also if the sensor is arranged at a slightly different location by a compensation procedure taking into account a standard deviation or the influences of the ambient conditions. It has been found that it is of utmost importance for the quality of the printed image that all parameters are adapted to the surface voltage of the image drum exactly at the depositing location of the toner on the image drum. Since the sensor detects the charge of the image drum at a predetermined angular distance from the first charging unit, which is exactly at the location where the toner is deposited on the drum, all previous influences can be eliminated in an advantageous manner by a corresponding compensation. It is, for example, possible in this way to compensate for a reduced surface voltage at the image drum instantly by a reduction of the voltage of the toner unit. At the same time, the voltage of the charging unit can be increased. When the area of the image drum, which was charged by the increased voltage of the charging unit, reaches the toner depositing location, that is, the sensor, the voltage of the toner unit can again be increased, since the previously lowered surface voltage of the image drum was completely compensated for by the increase of the voltage of the first charging unit.
Particularly advantageous is an embodiment of the invention wherein the image drum has an edge area which can be charged by the charging unit but which is not exposed to the beam of the writing unit and to which no toner is applied by the toner unit, and the sensor is arranged so as to detect the electrical charge of the image drum in this edge area. With such an arrangement, the sensor can be easily arranged in a simple manner exactly at the same angular distance from the first charging unit as the toner unit.
If it is for example determined by means of the sensor that the surface voltage of the image drum decreased in the area of the toner unit, this influence can be compensated for by an increase of the voltage of the charging unit as it is provided for in a particular embodiment of the present invention. However, it is also possible, instead of increasing the voltage of the charging unit, or in addition thereto, to reduce the voltage of the toner unit as it is done in accordance with another embodiment of the present invention. If the voltage of the toner unit is reduced, it is expedient, also to adjust the voltage of the transfer unit by which the toner present on the image drum is transferred to the medium. It has been found to be particularly advantageous if, whenever the voltage of the toner unit is changed, also the voltage of the transfer unit is changed correspondingly.
It has furthermore been found to be expedient, also to change the voltage of a discharge unit, by which the charge of the image drum is erased, depending on the output signal of the sensor. For the case that the surface voltage of the image drum increases in the area of the toner unit, an increase of the voltage of the discharge unit has been found to be very advantageous.
It has furthermore been found that, upon an increase of the surface voltage of the image drum in the area of the toner unit, an increase of the voltage of the writing unit positively affects the image quality.
Further features of the invention will become apparent from the following description of a particular embodiment of the invention on the basis of the accompanying drawings.